Method and apparatus for producing pressed plates



W- HOPPELER Nov. 1, 1966 METHOD AND APPARATUS FOR PRODUCING PRESSED PLATES Filed July 17, 1962 INVENTOR WALTER HOPPELER BY .ffika/ ATTORNEY United States Patent 3,283,048 METHOD AND APPARATUS FOR PRODUCING PRESSED PLATES Walter Hoppeler, Hans Lampitschen, Klosters, Graubuenden, Switzerland Filed July 17, 1962, Ser. No. 210,524 Claims priority, application Switzerland, July 19, 1961, 8,46 1/ 61 8 Claims. (Cl. 264-113) The present invention relates to a method and apparatus for producing pressed plates and more particularly is concerned with a method and apparatus for producing pressed plates of wood particles and hardenable, preferably thermoplastic binder material.

According to conventional methods pressed plates, it has been assumed that the very small wood particles, and especially Wood dust which accrues during the manufacturing process cannot be incorporated into the pressed plates since such very fine wood particles were assumed to cause difiiculties in the manufacturing process and to impair the quality of the finished product. For this reason, screening devices are provided for screening out and discarding the most finely subdivided wood particles.

However, from an economic point of view, the d18- carding of the fine wood particles and wood dust represents a very wasteful and expensive procedure since the thus eliminated wood portion amounts, depending on the type of wood and choppers used in comminuting the wood, to between about 3 and 10% of the wood initially introduced into the process. Furthermore, these screenedout line wood particles constitute a waste product for which it is difficult to find other uses. In addition to the wood dust and very fine particles which accrue during the production of the pressed plates, a further quantity of Wood dust is formed during the grinding of the finished lates. Since conventionally the cross-sectional dimensions of the plate are reduced by grinding to the extent of between about 0.5 and 1 mm. on each face of the plate, and since the outermost portion of the plates which are ground consist of a more dense material than the core portion of the plate, it follows that the amount of dust formed upon grinding of the plates will equal between about 5 and 20% of the total weight of the plates, depending on the thickness of the same.

According to the conventional methods of producing pressed plates, the final wood particles and particularly the wood dust absorb a higher proportion of the binder material than the larger wood particles. This is due to the fact that the absorption of binder material is substantially corresponding to the available surface of the Wood particles. In other words, since smaller wood particles have a relatively larger surface area than larger wood particles, more glue or binder material will be absorbed per unit of volume by smaller or dust-like Wood particles than by larger wood particles. The smaller the wood particle, the larger will be the surface area per unit of volume and thus, the larger will be the amount of binder material absorbed by the wood particles. Since conventially a given relationship is maintained between the amount of binder material and the amount of wood particles, i e. the amount of absolutely dry wood in the mixture, it follows that the strength of the pressed plate will be reduced if the same contains a relatively larger proportion of very fine wood particles. This is due to the fact that a disproportionate amount of binder material will be absorbed by the smaller wood particles and thus only an insufiicient amount of binder material will be available for the larger wood particles. This will reduce the quality of the pressed plates. On the other hand, if it is desired to maintain a predetermined quality of producing Patented Nov. 1, 1966 ICC of the pressed plate, then it would be necessary to increase the amount of the relatively expensive binder material somewhat in proportion with any increase of finer or dust-like wood particles in the mixture of which the pressed plate is formed.

It has been proposed to sift the wood particles during formation of the layer thereof which is then pressed to form the pressed plates, so that a plurality of superposed layers will be formed each of which will consist of wood particles of substantially equal size and whereby the size of the wood particles will change from layer to layer. However, by forming a pressed plate of such superposed layers which differ relative to each other with respect to the size of the wood particles, the binding of the plate and thus the coherence of the same is impaired since the main quantity of the binder will be in the layers formed of finer wood particles, i.e. in the outer layers of the plate, while the coarser wood particles in the core portion of the plate will contain only an insufficient proportion of binder material and thus will not possess the desired strength characteristics. Thus, by proceeding in this manner one will either have sufficient binder material in the outer portions of the pressed plate and an insuflicient amount of binder material in the core portion, or the core portion of the .plate will contain a sufiicient amount of binder material and in this case the outer portions of the plate will contain an excessive amount of binder material.

The excessive amount of binder material which thus might be present in the outer portions of the plate, i.e. the portions which according to this method are formed of Wood dust and finely particulate wood, will not increase the strength of the plate and, furthermore, during the grinding of the pressed plates, the outermost portions which contain most of the binder materialwill be ground off and thus no longer form part of the plate. In addition, the hard binder layer formed in this manner in the outer portion of the plate causes great difficulties during the planing and in any event excessive wear of the cutting or grinding tools. Furthermore, the bending strength or rigidity of the plate is impaired by having excessive proportions of wood dust and finely particulate wood in the outermost portions of the plate.

For all these reasons, up to now, conventionally at least a portion of the wood dust and finely subdivided wood particles are screened out and, to the extent that a portion of the wood dust and fine wood particles are retained in the mixture, it is attempted to influence in various ways the quantitative relationship between the binder material and the coarser and finer wood particles. However, these attempts have met only with very limited success and cause a reduction in the quality of pressed plates.

It is therefore an object of the present invention to overcome the aforementioned difficulties and disadvantages.

It is another object of the present invention to provide a method and arrangement for producing pressed plates which permits utilization of substantially all of the fine wood particles and wood dust accruing during the manufacture of the pressed plates, without imparing the quality of the plates and without causing an excessive consumption of binder material.

Other objects and advantages of the present invention will become apparent from a further reading of the description and of the appended claims.

With the above and other objects in view, the present invention comprises in a method of producing pressed plates by hot pressing a plurality of superposed layers formed of mixtures consisting essentially of wood particles of different size ranges and hardenable binder material, the improvement which consists in separating the wood particles which are to form at least one of the superposed layer into the different size ranges, forming a first homogeneous mixture of predetermined proportions of the separated wood particles of different size ranges, in-timately mixing the first homogeneous mixture with hardenable binder material so as to form a second mixture adapted to be hot pressed into a pressed plate, forming a plurality of superposed layers each consisting essentially of a mixture of wood particles and hardenable binder material, at least one of the plurality of superposed layers being formed of the second mixture, and subjecting the plurality of superposed layers to hot pressing so as to form a pressed plate thereof.

According to a preferred embodiment, the present invention contemplates in a method of producing pressed plates by hot pressing a plurality of superposed layers formed of mixtures consisting essentially of wood particles of difierent size ranges and hardenable binder material, the improvement which consists in separating the wood patricles which are to form each of the superposed layers into the different size ranges, continuously forming a plurality of homogeneous mixtures of predetermined volumetric proportions of the separated wood particles of different size ranges, each of the plurality of homogeneous mixtures being adapted to form one of the superposed layers of the plurality of superposed layers, drying the thus formed homogeneous mixtures, respectively, intimately mixing a predetermined volumetric proportion of each-of the thus dried homogeneous mixtures, respec-' tively, with hardenable binder material soas to form a plurality of latently adhesive mixtures adapted to be hot pressed into a pressed plate, forming without sifting of the plurality of latently adhesive homogeneous mixtures a plurality of superposed layers, and subjecting the plurality of superposed layers to hot pressing so as to form a pressed plate thereof.

The present invention also includes in an arrangement for producing pressed plates, in combination, separating means for separating wood particles into different size fractions, storage means operatively connected to the separating means for storing the thus separated fractions of wood particles, and mixing means for mixing predetermined proportions of the respective stored separated wood fractions so as to form thereof a homogeneous mixture of wood particles of predetermined size distribution.

Thus, according to the present invention it is possible to utilize wood dust and finely particulate wood material which up to now could not be used or would have caused either impairment in the quality of the pressed plates or the consumption of an excessive amount of binder material.

The problems discussed further above are solved according to the present invention by forming at least one of the layers of which the pressed plate is made of a substantially homogeneous mixture of wood particles of various sizes which mixture contains predetermined quantities of the various size ranges. This is preferably accomplished by first sorting the wood particles into separate size ranges and then to withdraw predetermined proportions from the storage hopper or the like for each of these size ranges and to form an intimate mixture of the thus withdrawn proportions. The binder material is then admixed to this mixture of predetermined proportions of material, a layer is formed of this mixture and is then subjected to hot pressing as will be described in more detail further below. The thus formed plate will consist of a relatively small number of layers each of which will consist of a homogeneous mixture of wood particles of various sizes, whereby the individual sizes or size ranges of the wood particles will be present in predetermined proportions. r The arrangement in which the above described metho is carried out will include sifting, separating or classifying means for separating wood particles into several size 4 ranges, storage means for storing the thus separated wood fractions, and means for withdrawing predetermined amounts of the individual wood fractions from the storage means therefor, and for mixing these predetermined fractions or proportions.

Increasing difficulties in obtaining desired wood qualities force producers of pressed plates to use more and more of relatively low wood qualities and also to use in the place of one wood type a mixture of wood of various origin, as well as wood dust, saw dust and chips and other broken wood obtained from other operations.

It is of greatest importance for the producer of pressed plates to be in a position to offer pressed plates of consistently even quality, in other words, to produce pressed plates which vary as little as possible with respect to their strength characteristics, thickness, moisture content, surface quality, weight and dimensions, and with respect to which the consumption of binder material and wood particles remains as constant as possible. These requirements are progressively more difficult to fulfill since. it becomes more and more difiicult to obtain predetermined desired types of wood and Wood qualities and dimensions and it becomes more and more a practical necessity to produce pressed plates from just whatever types of wood and wood qualities and dimensions are available. The greater the variations in the raw material, the harder it is to fulfill the requirement of even quality of the finished pressed plates. Thus, the raw material must be sorted by qualities and dimensions prior to being introduced into the manufacturing process and the proportions of various qualities quality and dimensions are then comminuted, sifted into several fractions or size ranges of the thus produced chips and then stored-in individual storage containers or hoppers each of which will hold o'hips of even wood quality and substantially even size. Chips which were bought as such or which accrue in the production of pressed plates are also comminuted and classified and then stored in the respective storage containers assigned for such quality and size. It is desirable that the wood is treated with respect to its moist-ure content in such a manner and so stored that the moisture content of the wood will be within relatively narrow predetermined limits. Practically it is not possible up to now to arrive at a constant moisture content of the wood in an economical manner so that it is necessary to carry out a continuous and accurate control of the moisture content of the chips which are to be processed into pressed plates.

Certain other parameters, such as the distribution of chip sizes also cannot be maintained unchanged over long manufacturing periods.

However, by storing the chips according to size in separate hoppers and then mixing proportions of the various sizes of chips in a predetermined manner, itis possible to obtain a mixture in which practically all parameters can be maintained at constant values so that a predetermined mixture of chips is obtained which is macroscopically uniform and which will endow a finished pressed plate with the desired substantially unchanging qualities. The proportioning of the individual chip fractions or size ranges preferably is carried out by volume since the bulk weight or apparent density can be maintained. substantially constant and thus the weight of a given volume of the respective chip fraction will also remain constant.

However, it cannot be completely avoided that the moisture content of these controlled chip mixtures will vary and for this reason it is desirable to pass the chip mixture prior to admixture of the binder material over a scale which will immediately and exactly indicate any change in the moisture content of the chip mixture, since v essing of the chips.

such change in the moisture content does cause a variation of the volume weight of the chips. The thus determined undesirable variation in the moisture content of the chip mixture can then be rectified by proper control of the chip drier.

It is of primary importance according to the present invention that the proportion of wood and thus also the volume of the cavities or interstices in the chip layer Which is to be compressed will remain constant. This can be assured as described above by volumetric proportioning, whereby simultaneously a continuous moisture control of the mixture may be carried out. Such a moisture control hardly could be achieved in any other manner so that any deviations in the moisture content which might occur will become immediately apparent during the proc- The predetermined proportionin-g of the composition of the chip mixture, taking into considera tion the qualities of the various w ood types from which the chips are derived, as well as the size ranges of the chips, and the formation of an even, microscopically homogeneous chip mixture will result in the production of pressed plates of substantially unchanging quality.

It is thus possible, by proceeding as outlined above, to produce chip layers for compression into pressed plates which with respect to their composition, the properties of the wood thereof, the type of woods, the amount of binder material required and the dimensions of the various chip fractions of the mixture are practically even, so that samples of such chip mixture taken over prolonged periods of production, for instance day by day, will upon analysis show substantially identical properties and dimensions.

It is also part of the present invention, to determine the amount of chips to which a given quantity of binder material is to be added by volume and not by weight so that the influence of changing the specific weight of the wood and moisture content will be eliminated. This again leads to the production of pressed plates of very even quality and allows a very simple control and supervision of the chip mixture with respect to specific weight and moisture of the wood.

It is essential according to .the present invention that the chip mixture is so composed that the percentage of the various chip fractions or size ranges will remain constant and that within each size range the distribution of specific chip sizes also remain substantially constant.

It has been experimentally determined that an amount of binder material which remains constant for a given volume of a, uniform and unchanging chip mixture will result in even and substantially unchanging strength characteristics of the finished pressed plate.

According to the present invention, the very fine wood particles and dust .particles which absorb a relatively large proportion of binder material are used, so to say, as connecting elements for adhering the coarser chips to each other. Thus, the very fine wood particles and the wood dust which now is evenly distributed throughout the chip mixture will be used as a carrier of binder material and will make available such binder material upon hot pressing of the chip layer for binding adjacent coarser wood particles to each other.

Consequently, according to the present invention, the binder material-containing chip mixture is formed into layers, for subsequent hot pressing, without being sub jected to any sifting such as throw or wind sifting, inasmuch as it is desired to maintain in the chip layer an even and homogeneous distribution of the wood particles of various sizes.

Contrary to the above described prior art methods, a certain proportion of very fine wood particles and wood dust, which proportion may be even greater than the amount of such particles and dust which accrues during the manufacture, will have a favorableelfect, even apart from the economical advantages of cool utilization of the wood material.

As has been described above, the more finely particulate and dust-like rwood particles will absorb a relatively larger quantity of binder material as compared with coarser wood particles. For this reason, it is important to operate in such a manner that during the formation of the layer of binder-containing wood particles the homogeneous size distribution of the wood mixture will not be disturbed so that the very fine wood particles will be evenly distributed throughout the entire layer.

In this manner it is achieved that the very fine wood particles which contain an excessive amount of binder material will not interfere with the processing of the layer but, to the contrary, will improve the adherence of the larger wood particles to each other, since these finely subdivided binder or glue carriers facilitate even gluing or binding of the entire chip mixture of the layer without requiring excessive pressures and with a minimum of deformation of the individual chips.

The small, binder-carrying particles which are located in-between the coarser chips, so as to say, rivet the same to each other. Upon exerting pressure onto the entire chip layer, even the small binder-carrydng particles will be deformed and a binder layer will be formed which will connect adjacent and now abutting coarse chip portions. The foregoing is stated by way of explanation, without limiting the present invention to this specific explanation for the fact that in this manner pressed plates can be produced which combine a lesser specific weight with higher strength characteristics, or which, when produced to conventional specific weights and strength characteristics, require a considerably lesser amount of binder and also a lesser amount of wood raw material, due to the utilization of the fine and dust-like wood particles which previously were screened off.

It has then also been found according to the present invention, that it is not only possible to utilize the en tire quantity of finely particulate and dust-like wood particles which accrue during the production of pressed plates, but it is possible to introduce an additional amount of up to 25% and more of the weight of the finished pressed plate, in the form of finely subdivided wood particles, such as saw dust and wood dust which may be obtained from other sources. This latter feature represents a very substantial improvement in the economy of the production of pressed plates, since saw dust and the like is obtainable at considerably less cost than the conventional wood raw material. In this connection, it should be considered that usually the raw wood material cost amounts to about one third of the total production costs of the pressed plate.

In this connection, it is interesting to note that it has 7 been proposed previously to extend the glue or hinder material with organic and inorganic filler materials. The very finely subdivided wood particles and wood dust have an effect somewhat similar to that of these previously proposed filler materials, however, with the difference that the prior art filler materials had to be purchased separately and that the value and quality of the pressed plates was not improved by the same, while the glue or binder-carrying very finely subdivided Wood particles which according to the present invention are evenly distributed throughout the chip layer increase the percentage of wood substance in the finished plate.

It is even possible to mix coarse chips which do not contain any binder material with a sufficient quantity of finely subdivided binder material-carrying wood particles which then will give up suificient amount of binder material during the hot pressing of the thus-formed chip layer.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which the figure is a schematic fiowv diagram of an arrangement according to the present invention.

45, 46 and 47 into drier 11 in which the chips which are destined to form the outer layers of the pressed plates will be dried. Drier 11 includes a sifting device in which coarse, over-sized chips are separated from Referring now to the drawing, the wood arriving at the bulk of the material. These coarse chips are then sorting station 1 is first separated into the type of wood passed through conduit 48 into the processing 11ne II, which is to be used for the outer or cover layers of the while the dried chips which are to form the outer layers pressed plates and the type of wood which will serve of the pressed plates are passed to dry hopper 12.v Ifrorn to form the core or inner portion of the pressed late dry hopper 12, the chips are withdrawn in a volumetrically Each of the two groups of wood, namely the one which controlled even and continuous fiow. The chips withis to serve for the cover or outer layer and the one which drawn 'y QPP 12 are comblnfid Wlth Y further is to serve for the core portion of the pressed plates is Wood ffactlons, Ilamfily the y chips accnllng from then further sorted by wood type and quality. planers 28 and 29, and in accurately controlled propor- By way of example only, the sorting of the wood is tion of grinding dust obtained from grinding machines 32 shown in Table I. and 33.

TABLE I Distribution of Distribution by Shapes In Cubic Meters in Cubic Meters Wood Wood Types Types,

Outer Core Total Slabs and Split Round Layers Layer Splinters Wood Timber Pine-Spruce 10.5 13.0 Fir 13.5 9.0 Larch 1.5 Arve 2.5

Total Conifers 24.0 26.0 Slabs and Splinters 25.0

8.5 9.0 h 1. 2' MaplaLinden 1. 0 0.

Total Deciduous Wood 6.0 19.0 25.0 4.0 21.0

Total Mixture 30.0 70.0 100.0 25.0 41.0 34.0

As shown in the drawing, the wood destined to form the outer portions of the pressed plate =is processed in the portion of the installation indicated by I, while the.

wood destined to form the core portion of the plate is processed in the part of the installation indicated by II.

The diiferent wood portions, for instance as described in Table I, are then stored separately in storage areas 2 and 2a. Fro-m storage areas 2 and 2a, .wood bundles 3 and 3a in the desired proportion of the different qualities are then placed onto chain conveyors 4 and 411 on which the wood passes to band saws 5 and 5a. Band saws 5 and So out the wood to pieces of the desired length. The saw dust produced in this operation flows through conduit 39 in a continuous manner to processing line 11d of the line on which wood is processed for forming the core portion of the plates. The cut wood pieces pass on a conveyor belt to chip producing machines 6 and 611, wherein the wood :is cut into chips. Splinters formed during this process pass onto conveyor 10.

The outer layer-forming chips of processing line I pass from chip producing machine 6 to sifter 7 in which the mixture of chips is separated into large area, medium and fine chips. The large area chips are then further comminuted in mill 8. The medium size and fine chips, as well as the chips which have passed through mill 5 are then jointly passed through conduit 40 into sifter 41. Sifter 41 separates the chip mixture into a regular fraction which will not pass through a screen of 2 mm. mesh width but will pass through a screen of 4 mm. mesh width, and into a fraction of small chips which pass through a screen of 2 mm. mesh width. These separated fractions pass through conduits 42 and 43 into wet hoppers 9 and 44. From wet hoppers 9 and 44, chips are withdrawn in .the desired predetermined proportions. The controlled withdrawal or dosing of the chips from hoppers 9 and 44 may be carried out by volumetric or gravimetric measurement, however, as has been described further above, preferably, desired volumetric proportions of the difierent chip sizes are withdrawn from the wet hoppers and pass through conduits The chips from planers 28 and 29 pass through conduit 35 into dry chip hopper 12c, and the grinding dust through conduit 53 into dust hopper 12b. Dust hopper 12b has two delivery devices with volumetric delivery control.

The controlled volumes of wood components which are withdrawn from hoppers 12, 12b and 120 pass through a joint mixing conduit and from there into a volumetric measuring and dosing device 13 which controls volumetrically the flow of the wood mixture which is to form the outer layers of the pressed plate, for instance so that dm. /min. will be withdrawn from dosing device 13. The thus controlled volume of chip and dust mixture is conveyed over a continuous band scale 14 to binder-applying machine 15 in which a synthetic .resin powder, for instance a urea-formaldehyde powder in water is sprayed onto the wood mixture. The water of the synthetic resin suspension is absorbedby the dry chips and dust, while the synthetic resin powder remains as finely subdivided particles on the surface of the wood. The ,wood mixture will not become adhesive but only somewhat moister when passing through binder-applying machine 15.

The thus treated wood mixture which is to form the outer layers of the pressed plates will then be combined in layer forming device 17 with the mixture for forming the core portion of the pressed plates, which has been prepared in processing line II.

The wood material which is to serve for forming the core portion of the pressed plates is passed from storage 2a as wood bundles 3a to chain conveyor 4a and to band saw 5a. chine 6a, the chip mixture is produced and will pass, with the exception of splinters, to sifter 7a wherein the chip mixture is divided into fractions from which the coarser one .is passed through mill 8b. The splinters separated in chip producing machine 6a pass via collecting conduit 10 into splinter mill 811.

While in processing lines I and II round timber and split wood are processed which had been mixed in prede- In subsequently arranged chip producing ma-.

termined proportions, the processing line 11b which forms part of the overall processing line II serves for the processing of saw mill waste, i.e. of slabs, splinters and the like. These waste wood products are combined into bundles 3b and are passed directly into chip forming device 617 and from there with the exception of splinters formed during the chipping operation, into sifter 7b wherein again separation of two fractions takes place and the coarser fraction will be passed to mill 8b. The splinters formed in chip forming device 6b also will be passed via conduits 10 to splinter mill 8a.

The processing lines Ho and 11d which also belong to the overall processing line II, serve for the working up of chips and saw dust from outside sources. Depending on whether or not the chips obtained from outside sources are of more or less uniform size, a sifter may or may not the required. The two fractions of chips obtained from outside sources which are indicated by reference numerals 55 and 56 are passed in predetermined proportions relative to each other to sifter 70. In sifter 7c, the chip mixture is divided in three fractions, namely splinters, coarse chips and chips of desired intermediate size. The spinters from sifter 7c flow into conduit 1%) and into splinter mill 8a, the soarse chips are passed through mill 8c and the chips of desired size will reach the conveying device 49 which will also accept the chips from mills 8a, 8b and 8c, as well as the mill and fine fractions of sitters 7a, 7b and 76. Saw dust 57 is combined with saw dust from conduit 39 and passed through sitter 7d, from which the coarse fraction of the saw dust is conveyor to mill 8c. Conveying device 59 will pass the combined wood mixture into sifter 50 into which also the coarse particles from sifter 41 are introduced. The medium and fine chips from sifter 70. also are conveyed to sitter 50. Sifter t) separates the entire chip mixture into several fractions, for instance four fractions which are then passed into wet hoppers 9a9d.

Wet hoppers 9a-9d serve for conditioning of the chips and for converting the discontinuous input of chips into a continuous output. From wet hoppers 9a9d, the not yet dried flow of chips will pass continuously and at a preferably volumetrically controlled rate evenly through the further processing steps.

In the various wet hoppers are difierent chip fractions, for instance, for formin the core portion of the pressed plates, four fractions, namely saw dust, fine chips, regular size chips and coarse chips.

The dimensions of the chips in hoppers 9, 44 and 9a9d with respect to their size and the percentage amount thereof which is to be present in the final plate-rorming chip mixtures, are indicated by way of example in the following table.

form the predetermined chip mixture, i.e. a chip mixture which contains predetermined proportions of various chip sizes, and thus formed chip mixture is then passed to drier 11a. The mixture which is introduced to drier 11a will thus always have substantially the same bulk volume and in any given volume of the mixture, the total wood surface area will always be the same, due to the fact that the proportions by volume of coarse, intermediate and fine chips, i.e. the individual chip fractions will be the same due to the exact dosing of the amount of the chip fractions which are withdrawn per unit of time from hoppers 9a- 9d. i

The thus formed chip mixture is called a uniform moist chip mixture.

The coarse chips which are sepanated in the sitter portion of drier 11 of processing line I pass via conduit 48 into sitter 56 from there through the corresponding hopper onto mixing convey-or 16 and thus to drier 11a.

From drier 11a, the chip mixture flows into a mixing and dosing hopper 12a. From hopper 12a, the mixture is withdrawn continuously and at an even rate by volume.

The thus withdrawn mixture is combined with the also volumeitrically controlled flow of materials from dust hopper 12b. The thus combined flow of wood material now passes through a special volume dosing device 13a in which medium size chips are continuously and in exactly controlled volumetric proportions admixed to the uniform dry chip mixture. From there, the completed chip mixture flows over band scale 14a to binderapplying device 15a. The application of binder material and the subsequent formation of layers of the bindercontaining chip mixtures is controlled by volume.

Since the binder-applying machinery works continuously and with constant quantities of binder material per unit of time, it is possible by introducing a continuous even flow of the mixed chips, to achieve an even specific degree of binder application which is most important for maintaining an even quality of the finished pressed plates. The overflow chips from the dosing device 13a are returned to hopper 12a.

The chip mixture which is to form the outer layers of the plate passes over band scale 14 and the chip mixture for the core portions of the plate over a similar band scale 14a. These two plates serve to control and check whether the weight of chip mixture which passes within a unit of time remains constant, i.e. remains uniform. If any change in the weight of the chip mixture which passes per unit of time is noticed, then such variations in the weight indicate a variation in the degree of moisture of the wood or aschange in the specific weight of the wood raw material. Thus, the proper dry- TABLE I Chip Size Forming:

Hopper 9 Passing through 4 mm. mesh retained on 2 mm. mesln. by Weight of outer layers. Hopper 44 Passing through 2 mm. mesh Do.

Hopper 9a--. Passing through 8 mm. mesh-.- 30% by weight of core layer. Hopper 9b. Passing through 6 nun. mesh 20% by weight of core layer. Hopper 9c Passing through 4 mm. mesh..- 25% by weight of core layer. Hopper 9d Passing through 2 mm. mesh Do.

The figures given in Table II vary considerably depending on the type of chip-forming machinery, mills and also depending on the amount and quality of the chips from outside sources which are included in the chip mixtures. In order to obtain an even chip mixture for the core layer of the pressed plates, portions of the chips from hoppers 9a-9d are withdrawn continuously at an even rate and in a predetermined porportion relative to each other, which proportion may be determined by volume or by weight but preferably by volume. The chips for hoppers 9a-9d will flow at such predetermined rates onto a mixing band 16 on which the chips from the four hoppers are mixed to ing and the proper mixing of the components can be contnolled by observing the occurrence or nonoccurrence of weight changes on scales 14 and 14a. Due to the fact that according to the described process of the present invention the chip mixture remains uniform, changes in the specific Weight of the wood raw material and in the moisture content of the chip mixture will not have any effect on the quality of the plate with the exception of the specific weight of the same.

After passing through continuous binder-applying machines 15 and 15a in which a suspension of a synthetic resin powder, for instance of urea-formaldehyde resin in water is sprayed :onto the chip mixture, the respective chip mixtures flow to layer-forming'deVice 17 in which layers of the binder-containing chip mixtures are formed on metal sheets which pass through the layer-forming device. The chip mixtures drop as uniform mixtures without separation of particles by size or specific weight in the form of a thin veil over a width of for instance 1.75 meters on the moving metal sheets. It is important according to the present invention and contrary to prior ant methods, that no sifting occurs during the forming of the superposed layers which are to be compressed to a multilayer pressed plate. At first an outer layer 18 is formed on the moving metal sheet and then one or several core layers 19 and 20 and finally a top layer 21 which will form the other outer layers on the individual metal sheets pass then over scale 22 and thereafter pref era'bly are subjected to a first pressed plate. This portion of the process is described in more detail in my copending application Serial No. 194,449.

The crude pressed plates which are formed in heated multilayer press 24 are then immediately laterally trimmed to standard width in contact with automatic circular saw 25. The trimmings, i.e. the material which is cut oil the crude pressed plate is then immediately chipped in chip cutter 26. The thus obtained chips and saw dust are continuously passed through conduit 27 into the processing line H, i.e. into sifter 50.

The trimmed plates, while still warm, are then passed directly into planing machines 28 and 29 in which both faces of the plates are planed. Turning device 30 is arranged between planing machines 28 and 29. The chips which accrue during the planing of the warm pressed plates are passed through conduit 35 into dry hopper 12c and withdrawn therefrom in volumetrically controlled manner.

The planed pressed plates are placed into temporary storage 31 Where they are allowed to stay for several days. Thereafter, the plates are ground on both faces by means of grinders 32 and 33. Here too, turning device 30 is interposed between the two grinding machines. The grinding dust is passed via conduit 53 to dust hopper 12b. After grinding, the now finished pressed plates are placed into storage 34.

It will be seen from the foregoing that from the chip forming machine 6 to the layer forming device 17, two parallel processing lines I and II are utilized, whereby processing line I serves to produce the chip mixture for forming the outer layers of the pressed plates and processing line H is utilized for producing the chip mixture for the core portion of the pressed plate. Up to and including the dosing device in wet hoppers 9, 44 and 9a-9d, both processing lines I and II serve for producing uniform wet chip mixtures. However, the evenly maintained high quality of the pressed plates is specifically controlled in the most economical manner by producing the uniform d-ry chip mixture which is then proportioned exactly as desired in the volume dosing device 13.

From layer-forming device 17, the chip mixture for the outer and core layers of the pressed plates which now have been superposed as described above, are processed jointly on a so-called forming line up to planing machines 28 and 29.

Substantially identical equipment is used in processing line I and processing line II. Processing line I also may be subdivided into several parallel lines, somewhat similar to the center line of processing lines II, for instance under conditions where the type of starting material makes it advisable to thus subdivide processing lines 1.

Up to now, the hot pressed crude plates were first placed into storage 31 for cooling and the trimming and grinding was carried out after cooling for instance 7 days, whereby the grinding dust then could be used only for heating purposes or for sealing the cover layer. Due to introduction of planing machines 28 and 29, about half of the grinding dust loss is avoided since according to the described arrangement about 50% of the previously accruing grinding dust is now recovered in the form of useful shavings from the planing operation. However, also the grinding dust which will still accrue is returned into the process as valuable carrier of binder material.

The shavings which accrue in the planing operation are of such quality that they can be immediately used as part of the chip mixture for forming the out-er layers of the pressed plate. Consequently, the shavings accruing in planing machines 28 and 29 are continuously, for 'instance pneumatically, conveyed through conduit 35 into dry chip hopper 120.

The method as described above does not only permit to utilize the fine chip and ,dust fraction of between 2 and 10% of the finished plate weight which up to now formed waste material of the manufacturing process, but furthermore, the amount of binder material required according to the described method will be between 20 and 30% less than that required according to prior art processes. For instance, instead of 7.5 kg. of resin dry substance per kg. of wood dry weight as customarily used for forming the core layer, it suffices now to use about 5.5 kg. of resin dry substance. This in itself is of very great economical importance and causes a considerable reduction in the manufacturing cost.

In this connection it is interesting to note that the amount of binder material which will adhere to the finely subdivided chips and the wood dust equals about 20% of the dry weight of such small wood particles, while the larger chips hold only about 3% of binder material. For this reason, it is so important according to the present invention to \form the chip layer which is to be compressed of an intimate and even mixture of large and small wood particles and not to separate the components during formation of the layer in layer-forming device 17.

Depending on whether a larger or smaller number of different wood types or whether fully or partially debarked wood is used, it is possible to simplify or to further augment the above-described arrangement, i.e.. to treat larger or smaller number of wood qualities and chip sizes in separate devices.

The method and arrangement described herein, is based on the formation of a controlled uniform chip mixture. Already the various wood types which upon milling or chip production therefrom will give different average chip dimensions are kept separately. These various wood types or chip types include the regular chips which are produced from logs, round timber and split wood; the chips which are produced from saw mill waste such as slab and splinters and the outer portions of logs from which boards can no longer be produced; chips obtained from outside sources which are further milled; saw dust and the grinding dust which accrues in the grinding of the pressed plates.

The thus described method makes it possible to produce pressed plates of high and even quality in an installation which is technically simple and economical, and with a minimum of material and labor. Furthermore, it is possible as described above to arrange the processing steps in an easily controllable manner. The process takes into account and can he adjusted in accordance with the type of wood which is used, the specific weight of the wood raw material, the type and configuration of the chips which depend on the manner in which the same are produced, for instance by planing or in a chopper. The dimensions of the chips, the proportion of finely subdivided wood (particularly for the core layer), the even drying, even binder application and even layer formation, as well as the desired specific weight of the finished plate are taken into account as well as conditions which change during operation, such as the blunting of the chopper knives.

In the following Table III, maximum, minimum and preferred percentage amounts of the various wood sizes are described which have given good resultsfor insulating pressed plates, and pressed plates for furniture or build- TABLE III Chip Mixture for Cote Layer 3. In a method of producing pressed plates by hot pressing a plurality of superposed layers formed of mix- Between 2 and 4 Between 4 and 6 mm Between 6 and 8 mm Betvlgeen and 35% preferably by weight. I

0. Between 10% and preferably 20% by weight. Between 20% and preferlaby 30% by weight.

CHIP MIXTURE FOR OUTER LAYERS Maximum chip dimension:

Up to 2 mm .t Between 2 and 4 mm 'I hese figures depend on the kind of product required (insulation, furniture, building).

The example of preferred values is especially for a good pressed plate for furniture, the flakes being cut with a Bezner-Flaker.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. In a method of producing pressed plates by hot pressing mixtures consisting essentially of wood particles of different size ranges and hardenable binder material, the improvement which consists in separating said wood particles into portions each of which contains only wood particles of one of said different size ranges; withdrawing from each of said separated portions of difierent size ranges a quantity of each portion which is selected in such a manner that said withdrawn quantities of wood particles of dilferent size ranges, respectively, are in a predetermined proportion relative to each other; forming a first homogeneous mixture of said withdrawn quantities of separated wood particles of different size ranges; intimately mixing said first homogeneous mixture with hardenable binder material so as to form a second mixture adapted to be hot pressed into a pressed plate; and subjecting the thus formed second mixture to hot pressing so as to form a pressed plate thereof.

2. In a method of producing pressed plates by hot pressing a plurality of superposed layers formed of mixtures consisting essentially of wood particles of diiferent size ranges and hardenable binder material, the improvement which consists in separating the wood particles which are to form at least one of said superposed layers into portions each of which contains only wood particles of one of said different size ranges; withdrawing from each of said separated portions of difierent size ranges a quantity of each portion which is selected in such a manner that said withdrawn quantities of wood particles of dilferent size ranges, respectively, are in a predetermined proportion relative to each other; forming a first homogeneous mixture of said withdrawn quantities of separated wood particles of different size ranges; initimately mixing said first homogeneous mixture with hardenable binder material so as to form a second mixture adapted to be hot pressed into a pressed plate; forming a plurality of superposed layers each consisting essentially of a mixture of wood particles and hardenable binder material, at least one of said plurality of superposed layers being formed of said second mixture; and subjecting said plurality of superposed layers to hot pressing so as to form a pressed plate thereof.

Betvlgeen 40% and 60% preferably 50% by weight.

tures consisting essentially of wood particles of different size ranges and hardenable binder material, the improvement which consists in separating the wood particles which are to form each of said superposed layers into portions each of which contains only wood particles of one of said different size ranges; withdrawing from each of said separated portions of different size ranges a volumetric quantity of each portion which is selected in such a manner that said withdrawn volumetric quantities of wood particles of different size ranges, respectively, are in a predetermined volumetric proportion relative to each other; forming a plurality of homogeneous mixtures of withdrawn volumetric quantities of said separated wood particles of different size ranges, each of said plurality of homogeneous mixtures being adapted to term one of said superposed layers of said plurality of superposed layers; drying the thus formed homogeneous mixtures, respectively; intimately mixing a predetermined volumetric proportion of each of the thus dried homogeneous mixtures, respectively, with hardenable binder material so as to form a plurality of latently adhesive mixtures adapted to 'be hot pressed into a pressed plate; forming of said plurality of latently adhesive homogeneous mixtures a plurality of superposed layers; and subjecting said plurality of superposed layers to hot pressing so as to form a pressed plate thereof.

4. A method according to claim 3, including the step of continuously weighing said predetermined volumetric proportions of said dried homogeneous mixtures prior to mixing of the same with hardenable binder material.

5. In a method of producing pressed plates by hot pressing aplurali-ty of superposed layers formed of mixtures consisting essentially of wood particles of diiferent size ranges and hardenable binder material, the improvement which consists in separating the wood particles which are to form each of said superposed layers into portions each of which contains only wood particles of one of said difierent size ranges; withdrawing from each of said separated portions of different size ranges a volumetric quantity of each portion which is selected in such a manner that said withdrawn volumetric quantities of wood particles of different size ranges, respectively, are in a predetermined volumetric proportion relative to each other; forming a plurality of homogeneous mixtures of withdrawn volumetric quantities of said separated wood particles of diiferent size ranges, each of said plurality of homogeneous mixtures being adapted to form one of said superposed layers of said plurality of superposed layers; drying the thus formed homogeneous mix-tures, respectively; intimately' mixing each of the thus dried homogeneous mixtures, respectively, with hardenable binder material so as to form a plurality of latently adhesive mixtures adapted to be hot pressed into a pressed plate; forming without sifting of said plurality of latently adhesive homogeneous mixtures a plurality of superposed layers; and subjecting said plurality of superposed layers to hot pressing so as to form a pressed plate thereof.

6. In an arrangement for producing pressed plates, in combination, separating means for separating wood particles into different size fractions; storage means operatively connected to said separating means for storing the thus separated fractions of wood particles; withdrawal combination, separating means for separating wood particles into different size fractions; storage means operatively connected to said separating means for storing the thus separated fractions of wood particles; drying means for drying the stored separated fractions of wood particles; withdrawal means for withdrawing from said storage means a quantity of each of said different size fractions, respectively, which is selected in such a manner that the thus-withdrawn quantities of wood particles of different size fractions, respectively, are in a predetermined proportion relative to each other; and mixing means for mixing predetermined proportions of the respective dried separated-wood fractions so as to form thereof a homogeneous mixture of wood particles of predetermined size distribution.

8. In an arrangement for producing pressed plates, in combination, separating means for separating wood particles into different size fractions; storage means operatively connected to said separating means for storing the thus separated fractions of wood particles; drying means for drying the stored separated fractions of wood par,- tioles; withdrawal means for withdrawing from said storage means a quantity of each of said different size fractions, respectively, which is selected in such a manner that the thus-withdrawn quantities of wood particles of difiere ent size fractions, respectively, are in a predetermined proportion relative to each other; and mixing means for mixing said withdrawn quantities of the respective dried separated wood fractions so as to form thereof a homogeneous mixture of wood particles of predetermined size distribution.

References Cited by the Examiner UNITED STATES PATENTS 2,673,370 3/1954 Goss 264113 2,697,254 12/1954 Gordon 264113 X 2,700,796 2/1955 Roman 264-113 X 2,743,758 5/1956 Ushmann 264113 X 2,744,045 5/1956 Collins 264-113 X 2,746,895 5/1956 Duvall 264,-113 2,992,152 7/1961 Chapman 264113 X 3,011,938 12/1961 Ohapman 15-6-62.2 XR 3,098,781 7/1963 Greten 264113 X FOREIGN PATENTS 644,034 10/ 1950 Great Britain.

EARL M. BERGERT, Primary Examiner.

P. R. WYLIE, J. F. MATHEWS, W. E. HOAG,

Assistant Examiners. 

1. IN A METHOD OF PRODUCING PRESSED PLATES BY HOT PRESSING MIXTURES CONSISTING ESSENTIALLY OF WOOD PARTICLES OF DIFFERENT SIZE RANGES AND HARDENABLE BINDER MATERIAL, THE IMPROVEMENT WHICH CONSISTS IN SEPARATING SAID WOOD PARTICLES INTO PORTIONS EACH OF WHICH CONTAINS ONLY WOOD PARTICLES OF ONE OF SAID DIFFERENT SIZE RANGES; WITHDRAWING FROM EACH OF SAID SEPARATED PORTIONS OF DIFFERENT SIZE RANGES A QUANTITY OF EACH PORTION WHICH IS SELECTED IN SUCH A MANNER THAT SAID WITHDRAWN QUANTITIES OF WOOD PARTICLES OF DIFFERENT SIZE RANGES, RESPECTIVELY, ARE IN A PREDETERMINED PROPORTION RELATIVE TO EACH OTHER; FORMING A FIRST HOMOGENEOUS MIXTURE OF SAID WITHDRAWN QUANTITIES OF SEPARATED WOOD PARTICLES OF DIFFERENT SIZE RANGES; INTIMATELY MIXING SAID FIRST HOMOGENEOUS MIXTURE WITH HARDENABLE BINDER MATERIAL SO AS TO FORM A SECOND MIXTURE ADAPTED TO BE HOT PRESSED INTO A PRESSED PLATE; AND SUBJECTING THE THUS FORMED SECOND MIXTURE TO HOT PRESSING SO AS TO FORM A PRESSED PLATE THEREOF. 